研究顯示，摻雜劑、氧空位及應(yīng)力等因素對(duì)穩(wěn)定O相至關(guān)重要。盡管單一因素難以穩(wěn)定O相，多因素聯(lián)合作用被認(rèn)為是實(shí)現(xiàn)高性能鉿基薄膜的可行途徑。

來(lái)自廣東工業(yè)大學(xué)物理與光電工程學(xué)院蔣艷平副教授和西安電子科技大學(xué)周益春教授領(lǐng)導(dǎo)的團(tuán)隊(duì)，系統(tǒng)研究了氧空位（V_O）、單軸應(yīng)變和外部電場(chǎng)（E_e）對(duì)鉿基晶體能量的單獨(dú)及耦合影響。

Fig. 3 Effect of charged VO on crystals.

研究發(fā)現(xiàn)，雖然增加V_O的數(shù)量能減少鐵電相和單斜相之間的能量差距，但它并不能單獨(dú)確保鐵電相成為最穩(wěn)定的形態(tài)。由于自發(fā)極化（P_s）與V_O的濃度和電荷狀態(tài)密切相關(guān)，這一現(xiàn)象揭示了V_O與E_e之間的耦合作用。

Fig. 4 Effects of uniaxial strain and VO coupling on crystal energies.

當(dāng)氧空位與單軸應(yīng)變共同作用時(shí)，單軸應(yīng)變可獨(dú)立穩(wěn)定反鐵電相，并提升鐵電相的穩(wěn)定性。同時(shí)，V_O濃度和電荷狀態(tài)的提升可以減少為穩(wěn)定鐵電相所需的應(yīng)變。此外，單軸壓縮應(yīng)變能夠提高鐵電相的P_s，從而增強(qiáng)外部電場(chǎng)對(duì)相穩(wěn)定性的影響力。

這些發(fā)現(xiàn)指出，在鉿基材料中鐵電相的穩(wěn)定是一個(gè)典型的機(jī)械–電–化學(xué)耦合過(guò)程。當(dāng)同時(shí)考慮氧空位、單軸應(yīng)變和外部電場(chǎng)時(shí)，能夠更容易地實(shí)現(xiàn)鐵電相的穩(wěn)定。

Fig. 6 Effect of mechanical-electrical-chemical coupling on crystal energies.

該研究為解釋鉿基材料中的“喚醒”現(xiàn)象提供了新的解釋?zhuān)閷?shí)現(xiàn)及維持鉿基材料中的鐵電相提供了理論指導(dǎo)。該文近期發(fā)表于npj Computational Materials 9: 219 (2023).

Fig. 7 Phase diagram of hafnia.

Editorial Summary

Hafnium-based thin films have garnered attention for their compatibility with CMOS processes and impressive ferroelectric properties, yet the origin of their ferroelectricity remains unclear; it is generally believed to stem from the Pca21 space group phase (O-phase), which is inherently unstable in its natural state. Research indicates that factors such as dopants, oxygen vacancies, and stress are crucial for stabilizing the O-phase. Although a single factor may be insufficient to stabilize the O-phase, the combined action of multiple factors is considered to be a viable approach to achieving high-performance hafnium-based thin films.

Fig. 8 Phase diagram of HZO.

A team led by Prof. Yanping Jiang from School of Physics and Optoelectronic Engineering, Guangdong University of Technology and Prof. Yichun Zhou from School of Advanced Materials and Nanotechnology, Xidian University, systematically investigated the individual and combined effects of the V_O, uniaxial strain, and the E_e on the crystal energy of hafnia-based. The increase of V_O could reduce the energy differences between ferroelectric and monoclinic phase, but could not render the ferroelectric phase as the most stable one. Since the P_s shows a dependency on the concentration and charge state of V_O, it indicates that there is a coupling effect between the V_O and E_e. When both V_Oand uniaxial strain are present, the uniaxial strain can independently stabilize the antiferroelectric phase and promote the stabilization of the ferroelectric phase, and the increase in the V_O concentration and charge state reduces the strain demand in stabilizing the ferroelectric phase. In addition, the uniaxial compressive strain increases the P_s of the ferroelectric phase, which will enhance the effect of the E_e on the phase stability. These indicate that the stabilization of ferroelectric phase in hafnia is a typical mechanical-electrical-chemical coupling situation. When considering V_O, uniaxial strain and E_e simultaneously, it will achieve the purpose of stabilizing the ferroelectric phase easily. This work provides an explanation for the typical wake-up effect and theoretical guidance to obtain and stabilize ferroelectric phase in hafnia. This article was recently published in npj Computational Materials 9: 219 (2023).

原文Abstract及其翻譯

Mechanical-electrical-chemical coupling study on the stabilization of a hafnia-based ferroelectric phase (基于鉿氧化物鐵電相穩(wěn)定性的機(jī)械–電–化學(xué)耦合研究)

Fenyun Bai,?Jiajia Liao,?Jiangheng Yang,?Yanping Jiang,?Xingui Tang,?Qiuxiang Liu,?Zhenhua Tang?&?Yichun Zhou?

Abstract?The metastable polar orthorhombic phase is believed to be the origin of the ferroelectricity of hafnia-based films. The adjustment of stain, oxygen vacancies and dopant during film deposition and the wake-up electric cycling are common strategies to induce the ferroelectricity in hafnia. However, none of them could independently render the ferroelectric phase to be the most stable phase from the theoretical calculation results. The exact external conditions to stabilize orthorhombic phase still remain elusive. In this paper, we investigate the effects of the type, distribution, concentration, and charge state characteristics of oxygen vacancies and the uniaxial strain on the crystal’ energy, dielectric constant and spontaneous polarization (P_s); In addition, the impact of the applied electric field parallel to the P_s on the crystal’ energy is explored by first-principles calculations. It is challenging to independently stabilize the ferroelectric phase of hafnia-based films by a single component owing to the rather strict conditions. Surprisingly, the ferroelectricity can be easily obtained when simultaneously considering the effects of oxygen vacancies, uniaxial strain, and applied electric fields, suggesting the extremely important mechanical-electrical-chemical coupling effects. This work provides an explanation for the typical wake-up phenomenon in hafnia and a guidance for film applications.

摘要 鉿基薄膜的鐵電性被認(rèn)為源于其亞穩(wěn)態(tài)的極性斜方相。在薄膜的沉積過(guò)程中，調(diào)節(jié)應(yīng)變、氧空位和摻雜元素，以及“喚醒”的電循環(huán)，是促進(jìn)鉿基薄膜產(chǎn)生鐵電性的通行做法。然而，從理論計(jì)算來(lái)看，這些因素單獨(dú)作用都無(wú)法使鐵電相變?yōu)樽罘€(wěn)定的構(gòu)型。斜方相穩(wěn)定所需的確切外部條件至今仍不明確。本研究深入分析了氧空位的種類(lèi)、分布、濃度以及電荷狀態(tài)的特征，以及單軸應(yīng)變對(duì)晶體能量、介電常數(shù)和自發(fā)極化（P_s）的影響。此外，我們還利用第一性原理計(jì)算，探討了沿自發(fā)極化方向的電場(chǎng)對(duì)晶體能量的作用。由于實(shí)驗(yàn)條件的嚴(yán)苛性，通過(guò)單一因素來(lái)獨(dú)立穩(wěn)定鉿基薄膜的鐵電相頗具挑戰(zhàn)。出乎意料的是，當(dāng)氧空位、單軸應(yīng)變和外加電場(chǎng)的效應(yīng)同時(shí)作用時(shí)，鐵電性卻可以較為容易地實(shí)現(xiàn)。這一發(fā)現(xiàn)凸顯了機(jī)械、電學(xué)和化學(xué)耦合效應(yīng)的極端重要性。我們的工作為理解鉿基薄膜中常見(jiàn)的“喚醒”現(xiàn)象提供了新的視角，并為未來(lái)薄膜材料的應(yīng)用提供了寶貴指導(dǎo)。